Process for recovering molten phthalic anhydride



c. R. DOWNS May 18 1 948;

PROCESS FOR RECOYERING MOLTEN PHTHALIC ANHYDRIDE I 2 Sheets-Sheet 1\ Filed June 3, 1944 a 1 6a a 7 1 A t 0 7 m/ a 7 L g k 0 1 4 a 7 a 4 00M 0 m a 2a f 1 4 m 2, m a Z 4 IYNVENTOR @lmrlea R Down;

- c. R. DOWNS v PROCESS FOR RECOVERING MOLTEN PHTHALIC ANHYDRIDE Filed June 3, 1944 2 Sheets-Sheet 2 INVENTOR Charles P. Dom

' ATTO NEYS patented May 18, 1948 mooos's- FoanacovEm No-MoL-rm Pnrmuo ANHYDRIDE oharles-itt iziowns, 01a Greenwich, Conn: Applicatiomlunet 1944,.Serial No. 538,685

6 claims: (01. EGG-4215) l1 This invention relates totherecoveryofiphthalic anhydride after condensation from-vapor phase.

In the production of phthalieanhydride by, the catalytic oxidation-of. aromatic hydrocarbons, such as naph-thalene,. o-x-ylo1; phenanthrene etc.. in

the vapor phase; the-mixture-ot fiXed-g-ases-WateI Vapor and phthalic anhydride vapor and other partial oxidation products, discharged from. the. catalyst converters, is commonly cooled to they dew point of the phthal-icanhydridevapors and then condensed on cool. surfaces; commonlyin large box-like chambers frequently called hay barns. As the dew point of. this vapor-gasmixture is usually below themeItingLpQi-nt-ofphthalic anhydride, the separation otthe .phthalioanhydride from the gases ordinarily take i place by crystallization of the phthalic anhydride inlong thread-like crystals, which festoon thercondensing surfacesand the walls'of the chambers and seri ously impair the heat-transferrate; at saidrsurfaces. I

If allowedto accumuiate-on -the fioorsof the compartments, these crystalsi-orm a deep bed. similar in appearance to My They may be drawn out periodically by hand. rakes inserted throughdoorsin .the fjIOI'ltS- of the compartments to-buggies; ormechanioal'vconveyors mayform the bottoms of the compartments thepurposeof which is to remove the hay continuously as pro duced. Such operations-,mowever are ,attencb ed by serious healthhazards, The vaporsrof the phthalic and maleic anhydrides'and, the quinones are very irritating to mucous membranes. The gases also contain carboncmonoxider The methods of operation in common-use-result i-n=-egress of the vapors and gasesto the workingasp ace, through the opening of the doors or leakage around conveyors. This is dangerous the health of the operators Expensive-ventilating equipment-is therefore required.v Neither method for the removal of-the productsfrom the compartments is satisfactory, and withany such method the difilculty remains: ofmelting. such'-. a fluffy, heat-insulating materialt 7 When: phtha lic anhyd'ride; vapors condense out from-the accompanying gases, they crystallize not only in the form of. hay, butalso-to some extent as ahard adherent scale on. the walls of the compartments. This: layer of. scale also reduces the rate ofh'eat'transfer through-the walls. Various proposals have been-gmade to knock. or

scrape such: scale from the, walls by mechanical means, but the; wall areasare large, and such methods are unsatisfactory, Frequentcleani'ng of the walls is required, byactually sendingmen into the compartments with hand scrapers, again taking'proper precautionsagainsthealth hazards".

The 1 methods heretofore used for: unloadingth condensing; chambersiare excessively costly, from alabor standizaoint and,.moreover',they frequently inter-fere with production.

One object of the present invention, therefore,

is-to eliminate the health hazards heretofore in"- volvedin phthal'ic anhydride production.

Another object is to reduce operating costs.

Another object is to provide a simple method of removing the product from the condensing chambers which avoids unsatisfactory and costly mechanical equipment.-

A further object is to reduce the size of the condensing chambers by keeping the heat-removing-surfaces in an improved heat transfer condition. I

In. thi's specification. and the accompanying drawin s. I'chave.shown-andidcscribed a'preferred embodimentjof my invention and various" modifications thereof; it isfidbe' understood that these areanot intended, to be exhaustive nor" limiting ot the invention, but onithe' contrary are given for purposes of illustration in order that others skilled? in the. art may fully understand the invention and the principles thereof and the" man'- ner of applying it in practical use so that they may modify, and adapt itin various forms, each as maylbe-b'es't suitedito the conditions of aiparticularnse. v v V The invei'i'tidn W'llllbfe further dc'scl'ibedby ref= erence totheiaccompanyin'g, drawing in which Fig. 1' is a diagrammatic" view or flow sheet illustrating the method of my invention and apparatus suitable for the practice'thereof;;

Fig.2 isva diagrammatic perspective view of a condensing'an'd melting chamber;

Fig. 3"is a diagrammatic View in vertical section of'a modifie'd'formof condensing and melting chamber'taken on lin'e 3--3' o'f Figura and Figure ,4, is a plan view of the chamber similar to thatshown in Figure 3;

I The. gases containing the vapors of phthalic an'llyd'ride,v with other Icy-products, ifpresent, are cdoled'fs'omewhat below the" dew point of the phthal'ic anhydride' by' passing them through a chamber which iscooIeuito theidesired temperatureby any'suitable means; Although f'phtha'lic anhydritle is referred to ill thisspecific'ation' and tri a companyin'g claims, it will be understood that the invention willlb'e used most often in connectror'i with tlielimpurel solid such as collects in "h'a'y barns of a ph'th'alic anhydride plant". The term is, therefore,ito be construed to cover such impureproduct as well as the pure chemical, phthalic anhyd'rid'e- When the d'esildiamouht Ofphthalll'c' anTlY- dridefi'n the form of hay-like crystals has' acthat it will come intocontact with the phthalic commonly used for making phthalic anhydride.

In addition to this conventional (or any other suitable) system for making and condensing the phthalic anhydride, I provide accessory equipment for carrying out my process, as follows: i3 represents aheated storage tank, or other vessel, for holding a body of molten anhydride suitably above its melting point. A steam coil for heating the anhydride is shown at I2. I4 is a pump for circulating the molten anhydride, and line 56 having branches 5'! and58 provided with valves I5 and I6 are provided for discharging the ananhydride in the chamber, and wash it alongwith the molten stream. The total molten phthalic anhydride is then withdrawn to the supply source, or any other suitable point of disposal, or a, part may be drained, or pumped, from the bottom of the chamber. The temperature of the liquefied phthalic anhydride pumped into the chamber should be such that it will melt the phthalic anhydride and incidentally heat the chamber without itself solidifying, and the pumping rate must be adjusted to supply suflicient heat to melt the anhydride within a predetermined length of time. Suitable temperatures and pumping rates will dependv upon the quantity of condensed anhydride to be melted, and the size of the chamber. Heating means associated with the chamber as described below, will also afiect these factors.

By my method, the chamber may be made vaporand gas-tight, and there is no chance for egress of vapors or gases through the opening of discharge doors, or leakage around conveyors. The cost of pumping is negligible. No foreign matter is introduced as a contaminant. The time required for emptying the chamber may be very short. After the chamber has been discharged and cleaned by melting its contents according to my .process,the chamber may be cooled, if desired, by any suitable means, and it is then ready to perform again as a condenser for phthalic anhydride. V

In order to further illustrate my invention, attention is directed to Figure 1.

Numeral I indicates a compressor for supplying airfor the oxidation of the naphthalene raw material. A par-t of this air heated to a suitable temperature is supplied to vaporizer 4, at a suitable rate, regulated by valve 2. Naphthalene raw material is also supplied to the vaporizer at a suitable temperature and rate. Valve 3 is used to mix secondary air in proper proportion and temperature with the air-naphthalene mixture leaving vaporizer d. Numeral 5 indicates a catalytic converter of any suitable form, e. g., such as is described in U. S. Patent 1,604,739; and 6 is a heatexchanger to cool the products of the catalytic reaction to a desired temperature to relieve the heat-transfer load on the less efiicient cooling surfaces of the hay barn condensing chambers 9 and I9. These chambers 9 and ID are connected for alternate use by means of the valves land 8 and lines 49 and 59. The gases and vapors leave the chambers 9 and ID by the duct l I to any suitable disposal or recovery apparatus, which may be water-scrubbing towers, etc., not shown.

The above described parts of equipment are hydride into chambers 9 or ID. Manifold 45 and branch connections 46 and 41 provide for discharge of molten anhydride from the respective chambers, and valves 19 and 20 permit the bleeding ofi of the melted anhydride for further refining and disposition and the return to the vessel [3 of desired proportions.

A parallel train of equipment is represented in the drawings and identified by the same reference numerals with addition of. the letter a. The number of such interconnected parallel trains will be determined by engineering and operating considerations, and this second train, therefore, is representative of any desired number. Vessel I3, pump l4, and accessory pipe lines may be used for serving all or any groupof such trains; and likewise, condensing chamber In may serve as a diverting condenser for a considerable number of chambers 9, 9a, etc., with suitable connecting piping arrangements so that it can be substituted for a chamber 9 in any train while the chamber 9 is being cleaned of condensate as hereinbelow described.

The gases and anhydride vapors, for'example from converter 5a, may be led into chamber 9a, .while the gases and vapors from converter 5 are being diverted to chamber l0, and chamber 9 is being purged and cooled. Then the gases and vapors from converter 5 may be passed into chamber 9, and the gases and vapors from converter 5a led into chamber I0, while chamber 9a is being purged and cooled for reuse as a condensing chamber. The purging and cooling of chambers 9 and 9a requires such a short time by my process, that Ill may serve as a diverting chamher for these and other condensing chambers, and still leave time to purge chamber IU of the small amounts of anhydride collected therein per cycle. Or a series of condensing chambers, such as 9 and I0 can be used in rotation, with one being purged while others are in use, and eachafter purging being connected in lace of the one which has been longest in use, the latter being then cut out for purgingf i The design of the condensing chambers, 9, l0, and 9a, may be varied as to details of construction without departing from the scope of the invention. Figure 2 represents a perspective view of such a condensing chamber which, for example, consists of four straight-walls, 28 and 30, and a sloping floor M, and roof 43. The interior of the chamber is divided into two compartments by the vertical baflle 2 l, which extends from the roof of the chamber down to a convenient distance from the floor 4 I. The baflie 2| in the simple form shown may be a single sheet of metal or' other material. The floor 4| may be pyramidal in contour providing a hopper, so that liquid material can drain to the valved outlet pipe 31. The chamber is shown as supported by legs 32. r i g The walls 28 and 30, and roof 43 may be exposed to the surrounding, atmosphere for heat removal, or other cooling means may be provided.

#Ihe eases and vapors, :such as. :nhthalic anhimridewanoraenterthrough duct. 14.. pass dawnwardlv through compartment 125.6, and then upward through compartment 251 to the legit-duct zilfrom which-they may pass to other chambers or scrubbing towers.

r-Pi-pes :33 valued at :34 and provided with spray nozzles .35 serve to introduce superheated-molten phthalic .anhydride into the chamber and direct it against the Walls. and :roof and chamber contents ior melting solid anhydride previously condensed :in the form of hay hanging on the walls, or deposited on the floor, or in the form of scale-which has :formed .on previously cooled wailsproof-or ibaflie. The hot, molten, phthalic anhydride introduced through .33, plus the an hydride .melted in the chamber is discharged -illustrated by Figures 3 and. the condensation chamber may havebany of a variety of dormsand construction. The modifications include a plurality of spray pipes 33 provided with'nozzles 35 and manifolded in a header it provided with valve -The baiiie 2! is a solid sheet suspended from the roof 53. As shown, it {is preferable to provide a series of pipes 33 intheforrn of a grid or coils or concentric rings connected througha header 43, so that the sprays formed by the nozzles will wash the Walls of the compartments and fall directly down into the -hay" collected on the hopper-like bottom s l.

.A pump 14 and pipe line 56 for delivering moltenphthalic anhydride are connected to heade-rAfl. This pipe 56 andpump M are connected to a'source of molten phthalic anhydride I3 main- .tained-by suitable means at'a predetermined temperature as described above in connection with Fig. 1. A=valved outlet pipe is shown at 31 which may be connected to the return piping 45, 46 and 41, as shown in Fig. 1. The pipe line 56, the return piping 45, 46 and H, the header ts, the pump M and all intermediate connections are preferably steam jacketed and/or insulated, as shpwn at-48;, t vo s l dification between pureing operations or to melt out any solid which may have formed. Valved connections 44 are provided to admit an inert compressed gas or other purging fluid, from .a source not shown, after the valve 34 has been closed; and thereby foo-drive out any remaining phthalic anhydride :f-mmthespray pipes and connections and the nozzles 35. and thus to avoid clogging of these partsby solidification 0f the .phthalic anhydride.

All pertstof the apparatus are completely sealed from the atmosphere and it is unnecessary to useany mechanical removal devices whichmight develop leaks. Health hazard is thus avoided.

In using the chambers'shown in Figs. 2, 3,

.and accordingto the process of the invention,

whilezthedgasesand vapors are being introduced throughduct 24., the valves34 and the line 44 are. shut off. During that time, heat is dissipatedthroughthe walls by ordinary atmospheric 1 eventually drops of its own weight into hopper M where it'accumulates'as a hay-like bed. It also forms as a solid, adherent scale on the cooling siirirtcc .v radual y heat removal from. e sasesis cdu The ases coht,..in ns :a need -..amc nt -91" h thalic anhydride a e disc a sad th ou h duc 25 t t e chambers or-to cr bbe s- When the bedin the hopper .41, 0.1 the depos t on th cooling surfaces has huiltup toa :predeterm n d. de ree, the inlet and exit ducts .24 and 25 are closed, and the gases and vapor diverted :to other equipment describ d abov Melted, anhydride -is then discharged through nozzles 35 at. a temperaturesufiicientlyhigh to melt the b d the hopper, and the. scale on'the walls, and peimit, the molten anhyd ide. :to "be withdrawn through "3]. The'melting of the h dride on the previously cooled surfaces mayat fi fst be :delayedxby the low temperature =o.f-'tl1ese surfaces. As the flow of molten anhydride icon? tinues, however, suflicient heating occursrgtoicause fusion of all condensed phthalie anhydride. A ter the c am er has be n pureed .of'its co tents, th sprays. 35 are shut ofi-et :34 and izblowh ou by pe n t -=QQ T-: P SE a co nect o 4. .4 f r :a-short me t cutt .1.5 then c osed. the chamber is. cooled, and it is then ready for pe rm n t cond ns ng hmt ion y ct :2 amid h tem era u 9f the molten nhthalicanhydride which is circulated through the Smay pipes sh u d be sufi ie tly a ove" h me po n t supply he'heat of fusi n 0 the pht anhydride condensed in the chamber but also any heat losses during the purging operation and at the end of the nuhging the tempelh ilire of the condensing surface shougldv bev ificien' y above the melting point of thephthal ic ,a e so that the molten material can drain efiectively from those surfaces and leave a minimum iof the phthalic anhydride to crystallize upon further cooling. .If heat is supplied from other sources the. temperature and/or the volume of the re culated phthalic anhydride can be correspond.- insl-y reduced- The WhSbing-down of the walls by the sprays 5 is not o g at :labDrand time-sates thereby. reducing the amount of condoms g equipment required for u i ter u ted op ation of thecatalytic converters, but the surfaces can e kept c e f nhydride, and in a good heat transfer condition which. can-not beduplicatedby han o me nicalscnapicne. For t is pur se h a n m n o spr n s 3: ahdhoa les 3:5 as shown in Figs. 3 and 4 is desirable. Fur,- thermore, it is well known that a ,bed of phthalic .anhyd i is di i t melt, nd resumes along time if it merely rests upon a heating surface suchas a n e onalia ted melter. iiBy-lz v pro s the melting a s place e y apidlahecausexit is Washed by hot, liquefied 'anhydi ide, .ra h he do n to, and hrou h it. The rapid melt n of th anhv d in th ndens n cha ber elimin tes t nec ss ty to a e arate lter nt wh h th s l-id anhydri e s. coma sed.

H r. if the ahhydr de is c nd nsed .accordi g.toco eht onal pra ice ahdcharsed into an xt rna l ea e e l th e eric. prinri l hi m invention may appli d in m l i the condensed anhydrid by fl w n hot liquid .ah ydride-overthe rystalsin themelter, e..g; a melter equipped withsprays through which hot meltedanhydride isintroduced :to'meet the crys- :tals'therein.

Cooling jackets, or coils inside or outside o! the chamber, may be used to increase the heat the. Q!

exchange rate and thus further to increase the capacity of the condensing chamber. The provision and arrangement of such means are matters of engineering well understood in the art, but the use of cooling coils inside a phthalic anhydride condenser is made feasible by the present invention, since without it the deposition of scale on the cooled surfaces would be rapid and dimcult to remove by scraping. Because of the peculiarities of phthalic anhydride, however, I prefer notto place closely spaced cooling coils in a downward or horizontal flow of vapors, e. g. in compartment '26, because they would tend to prevent the hay-like anhydride from falling into hopper 4|, whereas in compartment 21, these hay-like crystals would form mostly on the lower side of the coils and fall off without forming a detrimental bed thickness on the top side of the coils.

I claim:

1. The method of recovering liquid phthalic anhydride which comprises passing gas comprising crude phthalic anhydride vapors through a condensing chamber wherein phthalic anhydride crystals will be formed, stopping said fiow of gas, impinging onto a mass of such crystals within said chamber molten phthalic anhydride heated to a temperature suificiently above its melting point to supply heat of fusion to said crystals, draining molten phthalic anhydride from said space, heating a portion of the molten phthalic anhydride thus withdrawn from said chamber to a temperature substantially above that at which it is withdrawn and spraying the phthalic anhydride thus heated back onto the remaining phthalic anhydride crystals to melt them.

2. In the manufacture of phthalic anhydride the process which comprises passing a crude phthalicanhydride vapor mixture through a condensing chamber and condensing the phthalic anhydride to solid form in that chamber, introducing into said chamber molten phthalic anhydride at a temperature sufficiently above its melting point to supply heat of fusion to said solid phthalic anhydride, contacting said molten anhydride with solid anhydride condensed in said chamber and thereby melting it, and withdrawing from said chamber-anhydride thus melted together with said introduced molten anhydride.

3. In the manufacture of phthalic anhydride by the catalytic partial oxidation of aromatic hydrocarbon in the vapor phase the process which comprises passing the products of oxidation through a condensing chamber substantially closed to contact with the atmosphere and con denSing the phthalic anhydride to solid form in that chamber, closing said chamber against flow of gases, introducing into said chamber molten phthalic anhydride at a temperature suificiently above its melting point to supply heat of fusion to said solid phthalic anhydride, contacting said "melting point to supply heat of fusion to said 8 solid'anhydride, contacting said heated molten phthalic anhydride with solid phthalic anhydride in said chamber and thereby melting it and withdrawing from said chamber phthalic anhydride thus melted together with said introduced molten phthalic anhydride.

5. In the manufacture of phthalic anhydride by the partial oxidation of aromatic hydrocarbon in the vaporphase the cyclical process which comprises passing the products of oxidation through a first condensing chamber, cooling said products therein and condensing the phthalic anhydride to solid form in that chamber, diverting the productsof oxidation to a second chamber, and condensing the phthalic anhydride to solid form in that second chamber, introducing into the first chamber molten phthalic anhydride at a temperature sufficiently above its melting point to supply heat of fusion to said solid phthalic anhydride, contacting solid phthalic anhydride condensed in said first chamber with said hot molten phthalic anhydride and thereby melting it, withdrawing from said first chamber phthalic anhydride thus melted therein together with said introduced molten phthalic anhydride, and again passing the products of said oxidation through said first chamber.

6. The method of recovering liquid phthalic anhydride from mixed gases including phthalic anhydride vapor which comprises passing the vapors over condensing surfaces cooled to' a temperature below the melting point of the phthalic anhydride within a continued space to deposit solid phthalic anhydride, in said space, discontinuing said flow and substantially sealing said space against escape of phthalic anhydride vapors and passing liquid phthalic anhydride at a temperature sufficiently above its melting point to supply heat of fusion to said solid phthalic anhydride into said space and over the surfaces of said solid phthalic anhydride to melt and substantially wash away the solid anhydride.

7 CHARLES R. DOWNS.

REFERENCES- CITED 'The. following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 277,012 Dinkel May 8, 1883 910,982 Allen Jan. 26, 1909 1,336,182 Andrews Apr. 6, 1920 1,369,729 Danckwardt Feb. 22, 1921 1,464,844 Downs Aug. 14, 1923 1,563,718 Brown Dec. 1, 1925 1,837,869 Jewett et al. Dec. 22, 1931 1,891,754 Daniels et al Dec. 20, 1932 1,891,891 Luft Dec. 20, 1932 2,019,112 Beekhuis Oct. 29, 1925 2,057,100 Jesperson Oct. 13, 1936 2,064,468 Foster Dec. 15, 1936 2,067,019 Riegler Jan. 5, 1937 2,076,033 Kniskern Apr. 6, 1937 2,112,024 Jarl et al. Mar. 22, 1938 2,160,177 Shuman May 30, 1939 2,215,968 Livingston Sept. 24, 1940 2,219,333 Rogers Oct. 29, 1940 2,247,910 Douglass et al July 1, 1941 2,302,888 Porter Nov. 24, 1942 FOREIGN PATENTS Number Country Date 622,727 Germany Dec. 5, 1935 

